Blends of about 70% nitrile rubber (NBR) and 30% polyvinyl chloride (PVC) are known to have excellent ozone and oil resistance. Various of these types of blends possess superior processing. A typical commercially available product that is such a combination is a nitrile rubber/PVC blend. Parts fabricated from such blends have a smooth finish and high gloss. Abrasion and oil resistance make them excellent polymer materials for shoe soles, hose, and for jacketing for wire and cable. These materials are also widely used in closed cell sponge applications.
Thermal stability of these blends is critically important to their performance when formulated into finished products. In addition, enhanced thermal stability will reduce the amount of off-grade material manufactured thereby reducing the production costs and increasing the product manufacturing efficiency.
It is understood by most practitioners in the art of compounding such materials that stabilizer systems for them should combine high temperature stability, good initial color and long term stability characteristics, and relative ease and economics of use. PVC resins have been stabilized with such materials as tin salts and zinc salts and combinations thereof. See, for example, U.S. Pat. No. 4,614,756 to Veldiserri. Combinations of aluminum salts and calcium salts have also been used as stabilizer combinations as has been disclosed by Scheidl et al in U.S. Pat. No. 4,008,193.
The stabilizers for a mixture of PVC and a nitrile rubber may comprise standard antioxidants and stabilizers including various amine and phenol antioxidants and other stabilizing materials. Typical of such stabilizer systems include sodium oleate, calcium stearate, zinc stearate, dioctyl phthalate, butylated hydroxy toluene (BHT), epoxidized soybean oil (ESO), and various barium-zinc salts.
Regardless of which stabilizer or stabilizer system is used, it is the current practice to add the stabilizer material(s), the NBR, and the PVC together. Among the problems that arise from this process include poor color stability and a need to maximize mixing time for better blending for blends of PVC and NBR.
These materials are mixed and fluxed in typical rubber machinery used for this purpose, such as a Banbury or Brabender Mixer. The instant invention, which differs from the usual practice by the prestabilization of the PVC, results in greatly increased mill time and reduced color. The ingredients comprising this blend are the same as those in current use; the marked and surprising difference in properties are the result of the process of prestabilization of the PVC.
The above remarks indicate the need in the art for new and improved stabilizer systems for polyvinyl chloride/nitrile rubber compositions. The improved stabilizer systems would impart the desired combinations of properties, including increased stability, as demonstrated by color stability and increased mixing time. An additional benefit of the stabilizer system of this invention would be increased efficiency in terms of time and expense of production of the PVC/NBR blend.
It is therefore an object of this invention to describe a process by which the PVC particulate and nitrile rubber is compounded with stabilizer and/or antioxidant in a high shear dry blending process. The shear should be sufficient to raise the temperature of the PVC particulate/stabilizer mixture to at least about 60.degree. C but no higher than 80.degree. C. The use of this pre-stabilized PVC particulate in the standard process yields a fluxed blend with significantly enhanced thermal stability leading to increased mixing (mill) time and better color stability.
It is an additional object of this invention to provide a high quality blended product with increased thermal stability that has a better (lower) color level than do blends not pre-stabilized according to the instant invention. It is a still further object of this invention to produce a blended material with increased mill or mixing time. It is an additional object of the invention to significantly reduce the amount of off-grade material produced under typical factory conditions without any extraordinary or costly quality control means.